During 9-11 the terrorists were able to disengage the transponders in the planes they hijacked - -What reason could there ever be to dis-engage transponders whilst in flight? -Why are transponders not permanantly on as are black boxes? Without transponder signal all the atc controller sees on screen is blip & maybe flight number NOT altitude & speed & other flight parameters or in the case of a declared emergency sqwark code 7700 - I have emailed this question to both FAA & CAA & await reply though I am not holding my breath

It is used in the process of one of the secondary ways to radar identify an aircraft. Also, when ATC deems that the secondary is not necessary upon the aircraft getting within 15 nm from destination airport, they may instruct to turn off the transponder. Also during certain phenomena such as ring around, ATC may instruct the pilot to turn off the transponder to reduce ring around or if there is a high population of actual targets (aircraft) to reduce scope clutter.

It is used in the process of one of the secondary ways to radar identify an aircraft. Also, when ATC deems that the secondary is not necessary upon the aircraft getting within 15 nm from destination airport, they may instruct to turn off the transponder. Also during certain phenomena such as ring around, ATC may instruct the pilot to turn off the transponder to reduce ring around or if there is a high population of actual targets (aircraft) to reduce scope clutter.

100% WRONG, in 30 years ive never ask to have a TXP turned off because i didnt think i needed the secondary target.do you even know what happens to an ARTS/STARS data block when the TXP is turned off ?...IT DROPS OFF AFTER 3 SWEEPS WITH WITH OUT A TXP SIGINAL

It is used in the process of one of the secondary ways to radar identify an aircraft. Also, when ATC deems that the secondary is not necessary upon the aircraft getting within 15 nm from destination airport, they may instruct to turn off the transponder. Also during certain phenomena such as ring around, ATC may instruct the pilot to turn off the transponder to reduce ring around or if there is a high population of actual targets (aircraft) to reduce scope clutter.

100% WRONG, in 30 years ive never ask to have a TXP turned off because i didnt think i needed the secondary target.do you even know what happens to an ARTS/STARS data block when the TXP is turned off ?...IT DROPS OFF AFTER 3 SWEEPS WITH WITH OUT A TXP SIGINAL

5−2−12. STANDBY OR LOW SENSITIVITYOPERATIONYou may instruct an aircraft operating on an assignedcode to change transponder to “standby” or “lowsensitivity” position:NOTE−National standards no longer require improved transponderto be equipped with the low sensitivity feature.Therefore, aircraft with late model transponders will beunable to respond to a request to “squawk low.”a. When approximately 15 miles from itsdestination and you no longer desire operation of thetransponder.b. When necessary to reduce clutter in amulti-target area, or to reduce “ring-around” or otherphenomena, provided you instruct the aircraft toreturn to “normal sensitivity” position as soon aspossible thereafter.PHRASEOLOGY−SQUAWK STANDBY,orSQUAWK LOW/NORMAL

It is not a requirement AT ALL, but for whatever reason it is in the book. Have I ever used it? Nope. I can't figure out why anybody would use it, but it is an answer for OP.

Transponders are normally turned off once the aircraft clears the runway after landing. It doesn't turn back on until the aircraft takes the runway on its next departure. Leaving the transponder on when taxiing is done at airports that have a ground radar system called ASDE-X, but otherwise that practice can cause issues for the approach/departure controller, especially if a radar (not used for ground radar) is located on or close to the airport.

Another practical use for turning off a transponder is if the Mode C altitude encoding component starts sending ATC bad altitude information. If the Mode C readout is less or greater than 300 feet from the pilot's assigned altitude, ATC will usually instruct aircraft to "stop altitude squawk." They then switch to using only pilot-reported altitudes, which can also be typed into the data block that corresponds to the aircraft.

It's important to remember that modern radar has two components: a primary radar and a secondary radar. Secondary radar is responsible for receiving and deciphering the data sent by a transponder, allowing the radar system to include the altitude information next to the aircraft's target, as well as receive the transponder code and include some flight plan data next to the altitude information. When a transponder is turned off or not equipped on an aircraft (some GA aircraft do not have transponders), ATC can still pick up the aircraft on primary radar. While a primary radar target does not have the altitude or flight plan information next to it, there are still methods of positively identifying a primary target. The hijacked aircraft on 9/11 were still visible on primary radar until they dipped below radar coverage, which in most places is at and above 3,000-5,000 feet.

How many of you have even seen ring around. ive seen it once in 30 years and it was in my USAF days at RAF Bentwaters using an old moble RAPCON. most of the FAA radars are now ASR 9/11s which are not prone to ring around.in the 5 years i was at C90 with all the traffic around KORD ive never seen it once. still cant think of one reason under normal operations to have a aircraft stop sqwk near an airport.

How many of you have even seen ring around. ive seen it once in 30 years and it was in my USAF days at RAF Bentwaters using an old moble RAPCON. most of the FAA radars are now ASR 9/11s which are not prone to ring around.in the 5 years i was at C90 with all the traffic around KORD ive never seen it once. still cant think of one reason under normal operations to have a aircraft stop sqwk near an airport.

At my facility, ring around is somewhat common but it doesn't last more than a few sweeps. It might be due to the fact it is an ASR 8.

How many of you have even seen ring around. ive seen it once in 30 years and it was in my USAF days at RAF Bentwaters using an old moble RAPCON. most of the FAA radars are now ASR 9/11s which are not prone to ring around.in the 5 years i was at C90 with all the traffic around KORD ive never seen it once. still cant think of one reason under normal operations to have a aircraft stop sqwk near an airport.

At my facility, ring around is somewhat common but it doesn't last more than a few sweeps. It might be due to the fact it is an ASR 8.

yep that one would do it i belive the 8s are still analog , dose it happen with air targets or when someone has the txp on while on the ground.are you using ARTS or STARS where you work?